The closest prior art is the U.S. Pat. No. 7,442,126, FIGS. 1, 2, 4, wherein, according the FIG. 2 of the present invention, a first shaft yoke 7 a second shaft yoke 8 and a control yoke 6 are pivotally mounted to each other to pivot about a common axis. The first shaft yoke 7 is pivotally mounted on a first shaft 2′ with the pivot axis perpendicular to the first shaft rotation axis, the second shaft yoke 8 is pivotally mounted on “a second shaft boss 3′ having an axis normal to said second shaft rotation axis”, as disclosed in the first claim of the closest prior art patent.
The control yoke 6 is pivotally mounted to the center of a spherical pantograph, or control, mechanism 9 to keep the common axis at the right orientation and the transmission ratio at 1:1. The one end 10 of the control mechanism 9 pivots about an oblique pin 11 of the first shaft 2′, the other end 12 of the control mechanism 9 pivots about an oblique pin 13 of the second shaft 3′.
The spherical pantograph mechanism 9 comprises six articulated “links” (they actually act as six articulated yokes), with all pivot axes intersecting at the center of the joint.
In total the closest prior art CVJ, or TCVJ (Thompson Constant Velocity Joint) comprises nine yokes of all kinds: a triad 5 that makes the main work and comprises a first shaft yoke 7, a second shaft yoke 8 and a control yoke 6, and six more links that constitute the control mechanism 9 that keeps the common axis at the right orientation.
Besides transferring torque from the one shaft to the other, the mechanism is capable for transferring axial load without needing thrust bearings, at least in the case the two shafts are in a straight line. When the two shafts are at an angle, thrust loads on the joint bearings result, just like in the conventional Cardan joint. The thrust loads are proportional to the axial load transferred by the joint and depend on the angle between the shafts and on the rotation angle of the shafts.
A disadvantage of the closest prior art (or TCVJ) coupling is that it cannot reliably operate with the two shafts at a straight line. A minimum angle of 2 degrees between the two shafts is required, otherwise “the parts of the coupling rapidly wear” according the inventor and maker of the TCVJ (quote from his web site, under the title Special Instructions: “Continuous operation of the TCVJ coupling at 0 degrees is not recommended as this will cause wear on bearings and cause damage to the coupling. For maximum efficiency and life of the TCVJ coupling, a minimum operating angle of 2.0 degrees is recommended.”)
With the two shafts at a straight line, the first shaft yoke 7 and the second shaft yoke 8 stay coplanar, which means their normal to the common axis pivots are in a straight line perpendicular to both, the common axis and the straight line along the two shafts. The support of the control yoke 6 becomes problematic and the result is the rapid wear of the bearings.
With the two shafts at an angle, the TCVJ transmits the torque of the driving shaft to the driven shaft and loads the control yoke 6 with a respective idle torque. In order the control mechanism 9 to support the control yoke 6 and to receive the idle torque, it bends slightly at one direction. After the shaft angle wherein the two shaft yokes become coplanar, the idle torque changes direction and the control mechanism 9 bends at the opposite direction. That is, in order to provide to the control yoke 6 the necessary support, the control mechanism 9 bends at one direction leaving its geometrically correct position, and when the necessary support changes direction, the control mechanism 9 bends at the opposite direction, leaving again its geometrically correct position.
Geometrically the TCVJ coupling is perfect, but the flexibility of the parts and the inevitable lash of the bearings spoil the geometry.
For instance, with just 0.02 mm lash in every bearing of the spherical pantograph of the TCVJ, and with the shafts of the coupling at one degree angle, the spherical pantograph 9 performs an oscillating motion bending, like a chord, initially at one direction until it is adequately bend away to provide the necessary support/force to the control yoke 6, then it bends at the opposite direction until it is adequately bend away to provide the necessary force to the control yoke 6 at the other direction, and so on.
The impact loads of this motion combined with the absence of rotation of the pin of the bearing relative to the rest bearing (because of the small angle between the shafts) spoils the lubrication of the bearing and causes the fatigue of the bearings.
Another disadvantage of the TCVJ is the moment of inertia of the control yoke that loads the driving and the driven system with an inertia torque and torsional vibrations. With the two shafts rotating at constant angular velocity, the control yoke rotates at a varying angular velocity. The bigger the angle between the shafts, the wider the variation of the angular velocity of the control yoke. The control yoke accelerates, absorbing energy from the shafts, and then decelerates, delivering energy to the shafts, two times per shaft rotation.
It is an object of the present invention to address the above disadvantages. Accordingly, there is provided a constant velocity joint as defined in the appended claims.